The invention relates to a spectroscopy method for nuclear pulses obtained at the output from a detector such as a photomultiplier detector having a scintillator crystal.
Such methods are used in numerous technical fields that require nuclear particles and radiation to be detected. This method applies, for example, to performing well logging measurements in a subsurface formation.
To perform such logging measurements, a tool is used which is displaced along a borehole passing through various different subsurface formations.
The tool performs physical measurements of the subsurface formation and transmits them after real-time processing to the surface, to means for performing further processing and to means for making use thereof.
Of the various technical measurements used in well logging, spectroscopy performed by means of a spectroscopic tool is used to determine the characteristics of the various constituent parts of the subsurface formation.
In general, the energy spectrum is obtained either by detecting natural .gamma. rays, or by detecting .gamma. rays emitted by a source and after they have interacted with the electrons of the constituent materials of such formation, or else by detecting .gamma. rays that result from interactions between the nuclei of the constituent materials and neutrons emitted by a source. A detector then serves to detect the radiation and to transform it into an electrical pulse signal whose amplitude is a measure of the .gamma. ray energy.
Further details may be obtained from the manual "Handbook on detection and measurement" by Glenn F. Knoll (1979) in which a nuclear pulse analyzer is described.
Reference may also be made to prior art constituted by European patent application No. 90 401 174.9 published under the number EP 0 396 464, in which a spectroscopic method is described as applied to a well logging tool for performing nuclear logging.
The method described in that prior art comprises filtering the analog signal constituted by electrical pulses supplied by the radiation detector in order to perform Gaussian type shaping. The method then comprises performing analog-to-digital conversion of the signal, followed by pulse detection by comparing the amplitudes of samples with a predetermined threshold.
That method also makes it possible, when so required, to change the baseline of the signal when the amplitude of a sample lies beneath said line.